Process the lime juice, zest, avocado, salt, and pepper in a mini food processor until smooth. Gradually add the oil until emulsified. Season the dressing, to taste, with additional salt and pepper.
Using a paring knife and working over a large bowl to catch the juice, remove the peel and white pith from the oranges. Cut between the membranes to remove the segments directly into the bowl. Squeeze any excess juice from the membranes directly into the bowl and discard the membranes. Add the watercress and dried cranberries. Toss with the orange juice and season with salt and pepper, to taste.
To serve: Spread 1 side of the jicama slices with a little of the dressing. Using the jicama slices as you would a tortilla, place the watercress salad in the center of each jicama slice and fold to make tacos.
Cook's Note: The dressing can also be made by hand. Use a fork to mash the avocado until smooth and add the lime zest and juice. Whisking constantly, drizzle in the oil until smooth. Season with salt and pepper, to taste.